If you actually want to make real income out of it, rather than just some pocket money to offset for your electronics habit, you may find it hard going.
If it's not a hobby then you need to have a workable net price and min orders on all your items, that or your shipping price has to be a lot higher than "the price the post office charges" do not underestimate the time it takes to package and ship items.
If somebody buys an LED for a pound, and it takes you 5 minutes - answer customer's question about "how bright is it", collect item from the cupboard, antistatic bag it, get a suitable sized shipping package, print out the address, invoice etc... then take to post office (especially if it's the only sale of the day!)... you can see how that's obviously unworkable .
When going for the hobbiest market, selling pre-made selections of things is a good way to improve this, and people do seem to like them - example, for a while I had a lot of different types of PCB header listed for sale, with very few takers, then I changed tactic and made up packages of what I thought would be a useful selection of pcb headers, and they started selling no problem and with a much better price:effort ratio. LEDs the same, a packaged selection of 50 leds in 5 colours is likely to sell better than just selling leds individually.
Get some high precision scales from ebay, "count" by weight not by fingers. Your precision scales need to read down to 0.01g for led sized items but also have some larger ones for big orders (higher the precision, less the total weight a scale will handle), if ever have to deal with loose surface mount or other really small parts, you will need even higher precision (0.001g). Add at least one piece more by weight for errors unless you have hand counted and verified at least once (even then, it's surprisingly easy to miscount multiple times and get the same answer!).
If selling SMD stuff on tape, measure don't count. Just draw on a piece of card a line which is the length of tape you need for X pieces, pull the tape to that length, cut it with a knife and a quick check for missing pieces that fell out.
Don't forget every direct cost involved in shipping, no just the postal charge. That anti static bag, that's about 5p, the packaging itself probably at least 20p , the return address label, the paper to print the invoice on, tape, gluestick, bubble wrap if necessary.... all adds up. Then there is your time, electricity, fuel and wear on the vehicle you use when you take it to the post office (if you can do this 'on the way' somewhere that's great, but you don't always have somewhere to be 'on the way' to)...
Then you have the items that go missing or get damaged in the post, or are reported faulty on arrival, or are reported as short-counted on arrival, which if you value your reputation you just have to suck up and send a replacement, so need to have those losses factored in.
And if you're importing from China especially, you will from time to time get the wrong thing, broken things, things which do not meet spec, things which were shipped poorly and you don't trust them... in short, you will get stuff you can't sell at all or can't sell profitably.
Not to mention things that you think are the ducks guts and were sure would fly off the shelves so you stupidly imported a heap of the things - only to find nobody else is interested in the slightest even when you slash the price to cost or below just to get rid of them after 6 months.